It is known that by means of expensive cathodic sputtering a large number of high-melting point compositions can be vaporized and condensed continuously in film coating plants in a process that lasts many hours.
However, no process is known for continuously evaporating unto a moving web a pore free coating of a simple chemical composition at high speed and in particular high-melting temperature inorganic compounds such as zinc sulfide, silicon nitride, titanium dioxide, silicon monoxide, chromium oxide and electricity conducting or non-conducting silicate glasses. Although there are several types of vaporization sources capable of vaporizing either molten or solid vapor material, the vaporization rate is low due to a small vapor emitting surface of the material being vaporized. The rate of vaporization of the high melting point material in the known processes is too low for continuous vapor deposition on a large scale where film speeds of up to 10 linear meters per second and higher are required. In addition, the vaporizer vessels conventionally used for accommodating the material being vaporized are quickly destroyed by contact corrosion. It has not hitherto been possible to vapor coat a single reel having 10,000 linear meters of film with a cohesive poreless layer of a composition such as silicon monoxide.
Expensive electron beam vaporization methods cannot be used since cavitation or implosion effects permit only discontinuous vaporization of compositions such as silicon monoxide. Even with the aid of a high-power laser as the energy source, in an apparatus in which vaporization is carried out in a cold crucible so that crucible reactions are prevented, it has only been possible to discontinuously produce films coated with silicon dioxide (see DE-A-16 21 306).
Another disadvantage of the known vaporization materials and vaporization apparatus for high-melting compositions is the often low quality of the deposited coating. Besides being very porous, the coatings do not adhere to the substrate since the vaporized molecules, in the best of cases, approach the substrate with a low kinetic energy corresponding only to the vaporization temperature. It has often been necessary to impregnate and strengthen the coating of silicon monoxide or silicon dioxide, for instance, with an additional layer of a compound containing silicone polymerized in a glow discharge.
Even in a careful vaporization from a resistance-heated vaporization vessel, it has been possible only at very low vaporization rates and by use of multiple vaporization chambers to prevent the formation of small holes that result by the perforation of the film by small solid or molten particles ejected from the vaporization vessel. In addition, known vaporization materials and apparatus, due to the low rate of coating formation because of the low vaporization rate and the crystal formation favored thereby, do not provide X-ray amorphous coatings. Amorphous coatings are of great importance, for instance, as additional layers for inhibiting gas and vapor diffusion in aluminum vapor-plated packing sheets, catalyst poison for autoregenerative capacitors, color and protective layers for hot stamping foils and other ornamental sheets, electronegative covers for edges of metal layers subject to spark erosion, capacitor insulators and weather-resistant protective coatings for aluminum vapor-plated radar screens.
In the known apparatus for vaporizing high-melting compositions such as SiO or ZnS, by means of a thermal radiation heat source, the material to be vaporized in solid form generally consists of a powder or a granulated material or a self-supporting block or mass. The thermal source of heat, usually a tungsten filament, is positioned in a cavity within the material to be vaporized. The heat source is generally in the shape of a hairpin and vaporized molecules escape from the cavity through an outlet opening (see U.S. Pat. No. 2,762,722).
In other embodiments, the material to be vaporized, in granulated form, is housed in a cylindrical container. The source of heat generally in the shape of a perforated pipe or spiral heated filament is vertically arranged in the container so that the vaporized molecules can escape upwardly through the cavity within the pipe or heated filament (see U.S. Pat. No. 3,129,315 and U.S. Pat. No. 3,153,137). Such an embodiment has become known in the literature by the designation "Drumheller apparatus".
An apparatus is also known wherein a powdery vaporizable material is present in a replaceable container situated within a heated cylinder. The heated cylinder is outwardly screened by heat by heat shields so that the vaporized molecules can escape only upwardly and can be additionally heated quickly by a second heat source in the form of a grid (see U.S. Pat. No. 3,104,178).
An apparatus in which the granulated vaporizable material is housed in a heated cylinder with a perforated inner wall having a tungsten filament positioned in the interior of the heated cylinder can also be operated with an additional energy supply in the form of an electron carried in a manner such that the vaporization can be effected not only by thermal radiation but also by electron radiation. A specially uniform vaporization and less undesirable sputtering of molten particles can be obtained (see U.S. Pat. No. 3,244,857).
Finally, an apparatus for the vaporization of high-melting metals such as chromium is known in which the vaporizable material is in the form of a compact bar mounted on a solid support. The bar and support are positioned within a source of radiation heat such as a hollow cylinder of tantalum plate (see U.S. Pat. No. 3,313,914).
In all the known apparatus, however, the vaporizable material is stationarily arranged in solid form independently of whether it is housed in a container as a powder or a granulated material or is compacted as a self-supporting solid body. The vaporizable material is coupled without contact with a source of heating radiation which is stationarily arranged. Such apparatus with a stationary arrangement of the vaporizable material and the source of radiation heat can be used in conventional vacuum vaporization equipment for the discontinuous vaporization of a few high-melting compositions in the amount required for a specific operation. The device can be operated to provide "splatter-free" vaporization with high kinetic energy of the vaporized molecules at a high vaporization speed. Apparatus of the known kind are not suitable for a continuously operated vacuum vapor-deposition operation in which a large amount of vaporizable material is required since the temperature of the source of radiation heat must be continuously increased as the vaporizable material is consumed to ensure uniform vaporization at a constant vapor pressure. The need for increased temperature leads to local overheating of the source of heat and often to premature destruction thereof. Even worse, some compounds, such as silicon nitride, will decompose on heating in vacuum into nitrogen gas and elementary silicon, the evaporation of the latter being retarded until most of the nitrogen gas has distilled off and pumped off by the vacuum pumps. But if the silicon nitride is being continuously fed towards the thermal radiation heater, the evaporation of silicon will be accompanied by the evolution of nitrogen gas originating from the silicon nitride not yet directly exposed to the radiation of the heater, so that the silicon directly exposed to and evaporated by radiated heat will re-absorb or getter nitrogen when condensing on the web or other moving target.
Also, many powder mixtures containing transition metals will on heating in vacuum form not one single, but a variety of compounds having different vapor pressures, so that the lower vapor pressure compounds will distill off last, and the result will be a non-homogenous condensed layer. Thus, when a mixture of 3 parts per weight of Cr.sub.2 O.sub.3 with 1 part per weight Si is heated in a static vacuum evaporation system, by a radiation heater heated to about 1800.degree. C., the compound first evaporating and condensing will be more of a dielectric than a conductor of electricity, but after the rate of evaporation has decreased and the heater temperature consequently increased to say 2000.degree. C., the condensed layer becomes gradually more conducting and optically absorbing. If however the same mixture is being continuously fed to the same radiation heater heated to say 1800.degree. C., the condensed layer will consist of a homogenous, electrically conducted glass.
The problem is to provide an apparatus for the vaporization of high-melting inorganic compositions by means of a photon-producing thermal source of radiation heat in a continuously operated vacuum vapor-deposition process, which apparatus makes possible the uniform "splatter-free" vaporization of the high-melting composition for coating a substrate from a large stock of vaporizable material, without premature destruction of the thermal heat source by local over-heating, or condensing layers of unwanted composition of non homogenous nature.